X-ray source



X-RAY SOURCE Filed Dec. 15, 1965 W M Adi-0L7 0,1,

mm), (1-11., Ma Q United States Patent 3,510,656 X-RAY SOURCE John Laurence Linsley Hood, West Monkton, Somerset,

England, assignor to British Cellophane Limited, Somerset, England, a British company Filed Dec. 13, 1965, Ser. No. 513,425 Claims priority, application Great Britain, Dec. 17, 1964, 51,430/ 64 Int. Cl. H05g l/00, 3/00 US. Cl. 250-84 14 Claims ABSTRACT OF THE DISCLOSURE This invention relates to an improved X-ray source. At present, the most widely used source of X-rays is the X-ray tube in which electrons, emitted from a heated filament, are caused to bombard a metallic target from which X-rays are emitted. One disadvantage of such an X-ray tube is that the output is not constant.

Recently, a novel source of X-rays, known as a Bremsstrahlung source, has been described, from which X- rays are emitted at a substantially constant intensity. In the Bremsstrahlung source, beta particles emitted by a radioactive material are caused to bombard a target from which the X-rays are emitted having a characteristic radiation spectrum depending upon the target material.

The radioactive material is normally chosen from a beta-particle emitting element which element or any decay product thereof does not emit gamma radiation or internal conversion X-rays, as these would tend to mask the desired X-rays.

One disadvantage of the conventional Bremsstrahlung source, particularly when the X-rays are of low energy (that is, an energy of less than about 20 kev.) is that the intensity of the X-rays is low, due to the substantial absorption by the solid target material of X-rays generated below the surface layer of the target. In the low energy sources, X-rays generated at a depth of two thousandths of an inch or more are substantially absorbed.

Another disadvantage is that the only practical low energy Bremsstrahlung sources developed as yet, consist of tritium, a beta-particle emitter, absorbed into a target material of zirconium or titanium from which only X-rays having the characteristic X-ray radiation spectral characteristics of these two materials are obtainable.

For certain purposes, it is desirable to have an X- ray source of substantially constant intensity and to have a wide variety of characteristic radiation spectra. For example, it is known to measure the thickness of a foil by interrupting a beam of low-energy X-rays emitted from a conventional X-ray tube and determining the amount of absorption of the X-rays by the foil. The amount of absorption varies with the thickness of the foil and consequently is a measure of the thickness of the foil. However, for accurate results, it is important that the X-ray source should be of a substantially constant intensity and for high sensitivity, when ice determining the thickness of thin sheet materials, the peak of the radiation spectrum of the X-rays used should preferably be as near as possible to the peak of the absorption spectrum of the material undergoing thickness measurement.

According to the present invention there is provided an X-ray source comprising a beta-particle emitting radioactive material or any decay product thereof does not emit gamma radiation or internal conversion X- rays, in the presence of a target material which emits X-rays when bombarded by the beta-particles, wherein the target material is in gaseous form.

In practice, the radioactive material and the gaseous target material are conveniently confined in a substantially diffusion-proof, X-ray and beta-particle impermeable container having at least one window formed from a substantially diffusion proof X-ray permeable, betaparticle impermeable, material to allow X-rays generated within the container to pass out to the exterior. Further, the radioactive material is preferably selected from those materials in which the peak energy value of the emitted beta-particles does not exceed about 0.175 mev. to ensure that the window of the container need not be inconveniently thick to prevent the escape of betaparticles from the container.

The radioactive material may be a solid, for example, Nickel 63 which has a beta-particle peak energy value (PEV of 0.062 mev. or Sulphur 35 which has a PEV of 0.167 mev. More preferably, however, the radioactive material is a gas since it can then be intimately mixed with the gaseous target material and thus the possible absorption of beta-particles by the radioactive material itself is substantially reduced. A suitable gaseous radioactive material is tritium which has a PEV of 0.018 mev.

Although, any radioactive material may be used in cooperation with any gaseous target material, there are practical limitations. For example, it is preferred to use materials which do not produce a corrosive reaction product which might subsequently attack the container. If tritium, for example, is used as the radioactive material and chlorine or bromine as the target material, hydrochloric acid or hydrobromic acid respectively could be produced which could attack a steel container.

In another form of the invention, the radio active material and the target material are chemically combined to form a gaseous compound. This ensures that the two materials are intimately dispersed one Within the other. Examples of such compounds are silicon tetrahydride, phosphorus trihydride, phosphorus pentahydride and hydrogen sulphide in which at least one of the hydrogen atoms is tritium. Alternatively, the radioactive material may be chemically combined with some other inactive material in order that the compound shall be of gaseous form, for example, hydrogen sulphide in which radioactive sulphur 35 is the radioactive material and sulphur is also the target.

The target material chosen in practice will depend upon the use to which the X ray source is to be put.

If the X-ray source is to be used to measure the thickness of an object such as a thin foil or other sheet material, then the target material which is chosen is preferably one having the peak of the X-ray radiation spectrum substantially corresponding to the peak of the absorption spectrum of the object. For example, with a mixture of tritium as the radioactive material and argon as the target material, a peak X-ray radiation spectrum at 4.2 A. is obtained. This combination is particularly suitable for use in the examination of an object containing chlorine which has a peak absorption spectrum at 4.39 A., and, in particular, for determining the thickness of a chlorine containing material such as polyvinyl chloride film or a vinylidene chloride copolymer coating on a relatively X -ray permeable base sheet, for example, a regenerated cellulose film or a polyolefin film.

The tritium-argon source may also be conveniently used to measure the sulphur dioxide content of a flue gas owing to the peak absorption spectrum of sulphur being 5.10 A. In another example, an X-ray source in accordance with the invention comprising tritium in admixture with gaseous sulphur dioxide has a peak X-ray radiation spectrum at 5.37 A. which may conveniently be used for detecting aluminium, for example, in the determination of the thickness of an aluminium foil, in which the peak absorption spectrum is 7.95 A.

Tritium substituted silicon tetrahydride emits X-rays having a peak radiation spectrum at 7.11 A., tritium substituted phosphorous triand penta-hydride emits X-rays having a radiation spectrum of 6.14 A. and tritium substituted hydrogen sulphide emits X-rays having a peak radiation spectrum at 5.37 A. Such sources, also, are useful for the detection of aluminium by absorption.

When more than one radiation peak or a broad emission spectrum band is required, a mixture of mutually non-reactive target gases having the desired radiation spectra may be employed.

The volume of the container required to enclose a Bremsstrahlung source, in accordance with the invention, having a useful practical activity may be quite small. For example, when using tritium, approximately 1 ml. of tritium with 2 mls. of gaseous target material confined in a 3 ml. space will have an activity of 2 to 3 curies. For higher activities larger enclosures are required.

When tritium is the radioactive material, a suitable container is formed from stainless steel inch thick having an X-ray permeable window of beryllium of from 5 to thousandths of an inch thick. Tritium has a high rate of diffusion through beryllium and thus, the thickness of the window must be sufficient to ensure that there is substantially no loss of tritium by diffusion, even at the expense of some absorption of X-ray radiation.

When the radioactive and target materials are chemically combined, the diffusion problem is not so acute due to the size of the molecules, so that thinner windows can then be employed.

The X-ray sources in accordance with the invention have the advantage that the different target materials which are employed give rise to X-rays having different X-ray spectral characteristics and thus a wider range of spectra characteristics are available than hitherto by selection of the appropriate gaseous target material. Furthermore, the intensity of X-rays from such a source may be fairly high in comparison with the known Bremsstrahlung sources, since the self absorption of the X-rays in the gaseous target is very much lower than when a solid target material is used.

The present invention still further includes an apparatus for determining the amount of an element or elements in a material, which apparatus comprises an X-ray source as herein described which generates X-rays absorbable by the element or elements and a detector, for example, a scintillation counter, for detecting the intensity of the X-rays after passing through the material and so determining the degree of absorption of the X-rays by the element or elements.

Further, the invention includes an apparatus for determining the thickness of a coating incorporating an X- ray absorbent element or elements applied to a film, which apparatus com-prises an X-ray source as described herein which generates X-rays absorbable by the coating and a detector for detecting the intensity of the X-rays after passing through the coating and so determining the degree of absorption of the X-rays by the coating and the thickness of the coating.

An apparatus for determining the thickness of a coating applied to film may conveniently be connected to a coating applicator so as to control the rate of application of coating composition to the film such that any variations in coating thickness detected by the thickness determining apparatus causes a compensatory change in the rate of application of coating composition to the film so that the thickness of the coating on the film remains substantially constant.

An example of an X-ray source in accordance with the present invention and its use in measuring the thickness of a coating on cellulose film will now be described with reference to the accompanying drawingsin-which:

FIG. 1 is a vertical cross-section of the X-ray source enclosed in a container and,

FIG. 2 is a diagrammatic elevation of the source being used to measure the thickness of a coating on a film.

Referring to FIG. 1, the X-ray source 1 consists of a cylindrical container 2. having walls 3 made of stainless steel inch thick and having as a lid on its upper end a window 4 made of beryllium 10 thousandths of an inch thick. A mixture 5 of 1 ml. of tritium (PEV of 0.018 mev.) and 2 mls. of argon (all measurements being at normal temperature and pressure) are confined inside the container which has an internal volume of 3 mls.

The tritium emits beta-particles which bombard the argon atoms with the consequent release of X-rays. The X-rays escape through the window 2 and may be directed as a beam on to any particular object as required. The peak of the characteristic radiation spectrum is about 4.2 A.

Referring to FIG. 2, the X-ray source described in FIG. 1 is used to detect continuously the amount of chlorine present in a coating of a vinylidene chloride copolymer on one side of a film 6 of regenerated cellulose, by absorption of the X-rays by the chlorine which has a peak absorption spectrum of 4.39 A. The thickness of the coatings can then be calculated. The absorption of the X-rays by the cellulose film is small.

The film of regenerated cellulose 6 weighs 32 grams per square metre and is coated on one side with a vinylidene chloride copolymer containing 91 parts by weight of vinylidene chloride and 9 parts by weight of acrylonitrile, (all parts by weight being based on the total weight of the copolymer) such that the final weight of the coated film is 36.5 grams per square metre.

The film 6 is continuously passed between the X-ray source 1 and a detector 7 consisting of a scintillation counter which detects the amount of X-rays passing through the film 6. The intensity of X-rays detected varies directly with the chlorine content of the coating and thus, is a measure of the thickness of the coating. A direct measure of the coating thickness is obtained by calibrating the detector 7 with a film 6 having an identical coating of known thickness.

The detector 7 may be suitably connected to the coating applicator (not shown) which applies the coating composition to the film 6 whereby variations in coating thickness detected by the detector 7 can give rise to corresponding increases or decreases in the amount of coating applied so as to maintain the coating thickness of the film substantially constant.

I claim:

1. An X-ray source, comprising;

a beta-particle emitting radioactive material, said radioactive material forming a decay product,

said material and said decay product being substantially free of gamma radiation and internally generated X-rays, and

a gaseous target material exposed to the beta-particle emission of said radioactive material for producing X-ray emissions.

2. An X-ray source as claimed in claim 1 in which the radio-active material is selected from a radioactive material in which the peak energy value of the emitted beta particles does not exceed about 0.175 mev.

3. An X-ray source as claimed in claim 1 in which the radioactive material is a solid.

4. An X-ray source as claimed in claim 3 in which the radioactive material is selected from the group consisting of Nickel 63 and Sulphur 35.

, 5. An X-ray source as claimed in claim 1 in which the radioactive material is a gas.

6. An X-ray source as claimed in claim 5 in which the radioactive material is tritium.

7. An X-ray source as claimed in claim 6 in which the target gas is argon.

8. An X-ray source as claimed in claim 1 in which the target material comprises a mixture of mutually unreactive gases.

9. An X-ray source as claimed in claim '1 in which the radioactive material and target material are chemically combined to form a gaseous compound.

10. An X-ray source as claimed in claim 9' in which the gaseous compound is selected from the group consisting of silicon tetrahydride, phosphorus, trihydride, phosphorous pentahydride and hydrogen sulphide in which at least one of the hydrogen atoms is tritium.

11. An X-ray source as claimed in claim 1 in which the radioactive material and the target material are con- -fined in a substantially dilfusion-proof, X-ray and betaparticle impermeable container having at least one window formed from a substantially diffusion-proof permeable beta-particle impermeable material to permit X- rays generated within the container to pass out to the exterior.

12. An X-ray source as claimed in claim 11 in which the container is constructed of stainless steel walls and said window is constructed from beryllium.

13. An X-ray source as claimed in claim 12 wherein said container has a volume of three milliliters and said radioactive material consists of one milliliter of tritium and said target material consists of two milliliters of argon.

'14. 'In a coating apparatus for applying to a film a coating having at least one X-ray absorbent element and including a source for emitting X-rays, a detector for determining the thickness of the coating when applied to the film such that any variation in coating thickness sensed by the detector causes a compensatory change in the rate of application of coating composition to the film so that the thickness of the coating on the film remains substantially constant, the improvement essentially consisting of said source being in accordance with claim 11.

RALPH G. NILSON, Primary Examiner S. ELBAUM, Assistant Examiner U.S. Cl. X.R. 

